1. Field of the Invention
The invention relates to a receiver using antenna beam forming for tracking a transmitter signal and a method for tracking a transmitter signal by using antenna beam forming.
2. Discussion of Background Information
Multipath propagation can be a relevant error source in positioning and navigation using a positioning and navigation system such as a GNSS (Global System for Satellite Navigation), particularly in areas with a high fraction of signal reflections such as in urban areas with large buildings. Due to multipath propagation, receivers for navigation signals transmitted by a positioning and navigation system, herein also called navigation receivers, receive reflected electromagnetic waves from transmitters such as satellites (indirect navigation signals), which can cause multipath interference with direct navigation signals. Such multipath interference limits the speed and accuracy of the position determining by a navigation receiver.
In order to overcome the problem of multipath interference, navigation receivers can reject received navigation signals with a low signal to noise ratio (SNR) since these signals are often indirect navigation signals having a much lower SNR than the direct navigation signals. However, this technique is not reliable since indirect navigation signals with a small incidence angle may have a SNR similar to that of directly received navigation signals.
Instead of detecting indirect navigation signals based on their SNR, special antennas can be used to “filter” indirect navigation signals. For example, signals from (NAVSTAR-)GPS (Global Positioning System) are clockwise circular polarized electromagnetic waves, which are changed into counter-clockwise circular polarized waves if they are reflected by metallic surfaces. Thus, antennas adapted to receive only clockwise circular polarized electromagnetic waves can be applied in order to reject indirect GPS signals reflected from metallic surfaces. However, indirect GPS signals reflected from non-metallic surfaces cannot be detected and rejected.
Another technique to reject indirect navigation signals is the application of highly directional antennas with a directional receiving pattern, which does not approximate the horizontal plane, thus not being able to receive indirect navigation signals with a small incidence angle. However, this technique has drawbacks for example in street canyons in cities since indirect navigation signals have in these areas an incidence angle comparable to that of direct navigation signals.
Instead of rejecting indirect navigation signals based on receiving techniques, signal processing methods may be used for an efficient rejection. For example, the PRN (Pseudo-Random-Noise) sequences of a received direct and indirect navigation signal can be compared. If the time difference of two compared navigation signals is larger than the clock period in the navigation receiver, the delayed navigation signal can be rejected. However, if the time difference of two navigation signals is smaller than the clock period, it can be difficult to decide whether to reject one of the navigation signals.
New techniques use antenna arrays in order to significantly improve navigation performances, in terms of multipath and interference rejection. The usage of an antenna array in a navigation receiver is one the most promising techniques to get rid of multipath problems, interferences and at the same time to have a very good signal to noise ratio. A navigation receiver employing an antenna array forms the reception beam of the antenna array towards the direction of a target transmitter of a navigation signal, particularly a GNSS satellite, in order to be able to track the direct navigation signal. The beam-forming process is based on the weighting of the received signal samples of each individual antenna array element with pointing direction dependent complex excitation coefficients (for amplitude and phase). If the pointing varies with the time, these excitation coefficients can vary accordingly. A GPS Receiver using a digital beam-steering antenna array to enable up to eight GPS satellites to be tracked is described in the publication “A GPS Digital Phased Array Antenna and Receiver”, Dr. Alison Brown, Randy Silva, Proceedings of IEEE Phased Array Symposium, Dana Point, Calif., May, 2000.